Tabular silver halide grains are crystals possessing two parallel faces with a ratio between the diameter of a circle having the same area as these faces, and the thickness, being the distance between the two major faces, equal to two or more.
Tabular grains are known in the photographic art for quite some time. As early as 1961 Berry et al described the preparation and growth of tabular silver bromoiodide grains in Photographic Science and Engineering, Vol 5, No 6. A discussion of tabular grains appeared in Duffin, Photographic Emulsion Chemistry, Focal Press, 1966, p. 66-72. Early patent literature includes Bogg U.S. Pat. No. 4,063,951, Lewis U.S. Pat. No. 4,067,739 and Maternaghan U.S. Pat. Nos. 4,150,994; 4,184,877 and 4,184,878. However the tabular grains described herein cannot be regarded as showing a high diameter to thickness ratio, commonly termed aspect ratio. In a number of US Patent Applications filed in 1981 and issued in 1984 tabular grains with high aspect ratio and their advantages in photographic applications are described. So Wilgus U.S. Pat. No. 4,434,226 discloses tabular silver bromoiodide grains having a thickness less than 0.2 .mu.m, a diameter of at least 0.6 .mu.m and an average aspect ratio greater than 8:1 and accounting for at least 50 percent of the total projected area of all the emulsion grains. Kofron U.S. Pat. No. 4,439,520 discloses similar grains which are spectrally sensitized. Abbott U.S. Pat. No. 4,425,425 describes radiographic materials containing tabular grains with an aspect ratio of at least 8:1 and Abbott U.S. Pat. No. 4,425,426 discloses similar grains with an aspect ratio between 5:1 and 8:1. A survey on high aspect ratio silver halide emulsions appeared in Research Disclosure, Volume 225, Jan 1983, Item 22534.
For radiographic applications the main photographic advantages of tabular grains compared to normal globular grains are a high covering power at high forehardening levels, a high developability and higher sharpness especially in double side coated spectrally sensitized materials. The thinner the tabular grains the greater these advantages.
In spite of these important advantages, tabular grains have two important disadvantages. These grains are highly susceptible to mechanical stress and the developed silver has an unacceptable reddish-brown colour compaired with the cold-black colour shown by more globular grains.
This reddish-brown colour can be corrected by increasing the optical density in the red region of the visible spectrum by adding suitable dyes to the undercoat layer, to the emulsion layer and/or to the protective layer. But this inevitably leads to an undesirable higher gross-fog of the photographic material and obviously the sensitivity to mechanical stress is not improved by this optical correction method.
A more convenient way to overcome these two disadvantages is to use tabular grains with an increased thickness. Methods to prepare these thicker tabular grains have already been described in U.S. Pat. Nos. 4,801,522; 5,028,521 and 5,013,641 but all these methods make use of ammonia or generate ammonia "in situ". Yet, methods using the volatile ammonia are difficult to control and are more susceptible to fog generation during the emulsion preparation.